Traditionally, it has been common to use wood such as plywood when there is a need for structural panels of relatively low cost. Increasingly, however, efforts have been made to develop low-cost structural panels from molded fibrous material such as wood fibers of the type used in some papermaking processes. In one process, an aqueous fiber stock is vacuum deposited on a porous mold or screen. The stock is partially dewatered and conforms to the shape of the mold to form a wet molded panel. The panel is removed from the mold and dried in a dryer to remove the water. In many cases, only the side of the panel that was against the mold is finished (i.e., smooth); the opposite side, which was not contacted by any mold surface, remains unfinished or rough.
More recently, processes have been developed wherein the panel is pressed between two mold parts so that both sides of the panel are finished. One such process is disclosed in U.S. Pat. No. 4,702,870 to Setterholm et al. The process produces a three-dimensional panel that is flat on one side and has a system of intersecting ribs similar to a honeycomb structure projecting from the opposite side. To make the panel, an aqueous fiber stock is deposited into a mold comprising a porous support plate or screen on which are affixed a plurality of resilient elastomeric mold pieces or projections of truncated conical or pyramidal shape. The mold pieces are spaced apart on the porous support plate so that intersecting channels are defined between the mold pieces. The aqueous stock fills the mold to a depth greater than the height of the mold pieces, so the stock covers the upper surfaces of the mold pieces. A flat mold plate is urged against the stock and presses the stock down into the mold; the stock is dewatered through openings in the porous support plate. The pressure on the stock in the urging direction causes compression and densification of the panel in its thickness direction (i.e., perpendicular to its plane). Moreover, this pressure also causes the elastomeric mold pieces to be compressed to a smaller height, and as a result they grow in width or diameter and thereby exert pressure on the stock in the lateral direction (i.e., parallel to the plane of the panel). Accordingly, the ribs of the panel that are formed in the channels between the mold pieces are compressed both in the thickness direction and the lateral direction. The process thus is able to produce a panel with substantially homogeneous density in all directions, and with substantial bending stiffness relative to its weight.
A drawback of the process of the '870 patent, however, is that the compressible mold pieces are not very durable and tend to break or become detached from the support plate after a relatively small number of molding cycles. Furthermore, the mold pieces tend to become compression-set so that they lose their ability to provide the needed lateral compression of the panel ribs. The compressible mold pieces thus must be replaced periodically, which is time-consuming and expensive.